1. Field of the Invention
The present invention relates to an overvoltage protection device which can prevent an electronic apparatus from being destroyed due to an overvoltage, such as a surge, caused by lightning, etc.
2. Description of the Related Art
Generally, an electronic apparatus is, when being used, supplied with a commercial power supply. In the case where lightning strikes a generating station, a transmission station, a substation, etc., a very high overvoltage (surge voltage) is momentarily applied to a power supply line through induction, sometimes causing the destruction of some element of the electronic apparatus. In the prior art, the electronic device has been protected by an arrester from such an overvoltage.
FIG. 1 is a block diagram showing a conventional overvoltage protection device using arresters. A power is supplied to an electronic device 1 by inserting an AC plug 2 of the electronic device 1 into an AC outlet 3 provided in a wall, a floor, etc., of a room. The AC outlet 3 has AC terminals 3a and 3b connected to commercial power supply lines (not shown) and a frame ground terminal 3c. The AC plug 2 has AC poles 2a and 2b connected to AC terminals 3a and 3b of the AC outlet and a frame ground pole 2c connected (grounded) to the frame ground terminal 3c of the AC outlet 3 through arresters 4 and 5.
In such conventional device, when any overvoltage occurs on a commercial power supply line and is applied to any one of the AC poles 2a and 2b of the AC plug 2, a corresponding arrester is operated (turned ON), so that short-circuiting occurs between the frame ground electrode 2c of the AC plug 2 and any corresponding one of the AC poles 2a and 2b of the AC plug 2 which is applied with the overvoltage. As a result, the overvoltage escapes to ground and no overvoltage is supplied to the electronic device 1.
The arrester has the disadvantage of being normally expensive and, further, its operation start voltage cannot be set at too high a level. Even if, therefore, any overvoltage is applied to the electronic device to a practically unaffected extent, a power supply to the electronic device is sometimes interrupted. This offers the disadvantage of the power supply being frequently interrupted to an unnecessary extent.
Since, as described above, the arrester is employed to protect an electronic device from an overvoltage such as a lightning surge, the overvoltage protection device involves high costs and, further, an unnecessary power supply stoppage has been encountered. The power supply stoppage while the electronic device being operated is not desirable and, sometimes, the electronic device functions abnormally.